1. Field of the Invention
The present invention relates to a projector lens system for projecting an image from a light bulb, especially a light bulb such as a digital micromirror device (herein after, DMD), which forms an image by changing directions of the micromirrors and the reflecting-direction of light rays.
2. Description of the Prior Art
In recent years, a DMD as a light bulb for a projector has become commercially viable, instead of liquid crystal display panels which have been widely used over many years. The DMD displays an image by arranging miniature micromirrors (mirror-surface elements) on a flat surface so as to correspond to pixels, and the angle of each mirror surface is mechanically controlled by employing micro-machine technology. Furthermore, the DMD has a faster response speed than a liquid crystal displays does, and can obtain a brighter image. Therefore the DMD is showing clear signs of wide popularization due to being suitable for achieving a miniaturized portable projector having high luminance and high image quality. As an example, Japanese Unexamined Patent Publication No.2001-51195 has taught an apparatus utilizing such a DMD.
In the case where a DMD is used in a projector apparatus as a light bulb, restrictions inherent to the DMD have to be imposed on the projector lens system.
The first restriction concerns the F-number of the projector lens system. Currently, when a DMD produces an image, the micromirrors are swiveled at an angle of ±12°; and by swiveling the micromirrors, effective reflection light rays (effective light rays) are separated from void reflection light rays (void light rays). Accordingly, in a projector utilizing a DMD as a light bulb, the following condition has to be met, i.e., it is required to use effective light rays only, and at the same time, not to use void light rays. By this condition, the F-number of the projector lens system can be determined, namely, the F-number is 2.4. Actually, since an increase of the amount of light rays as could as possible has been demanded, it is common practice to constitute the projector lens system having an F-number of 2.0, by considering a decrease of contrast within a range causing no adverse effects.
Furthermore, it should be understood that a condition like the above is established, provided that the light-bulb-side pupil of the projector lens system remains at a fixed position. Accordingly, in the case where the pupil position is variable, e.g., a zoom lens system, consideration has to be taken for optimizing the position of the pupil at the wide-angle extremity where brightness becomes generally lower due to loss in the amount of light rays.
The second restriction concerns the positional relationship between the DMD and the light-source system. In order to further miniaturize the projector lens system, it is preferable that the image circle of the projector lens system be as small as possible. Consequently, the position where the light source system, which inputs a projector light bundle to the DMD, is provided is limited, so that the light source system has to be positioned in substantially the same direction as the projector lens system (i.e., adjacent to each other) is, in order to make effective light rays from the DMD incident on the projector lens system.
Furthermore, the space between the light bulb and the most light-bulb-side lens element of the projector lens system (i.e., the back focal distance) is utilized for both the projector lens system and the light source system, so that a longer back focal distance has to be secured for the projector lens system, and at the same time, there is a need to make a lens system on the side of the light bulb smaller in order to secure sufficient light-guiding space from the light source system. This optical arrangement is equivalent to positioning the light-bulb-side pupil of the projector lens system in the vicinity of the rear of the projector lens system, from the viewpoint of the optical design for the projector lens system.
On the other hand, in order to improve the optical performance of the projector lens system, there is a need to use a large number of lens elements. However, if a large number of lens elements are employed in the projector lens system, the overall length thereof will become longer; and consequently, in the case of a lens system having the entrance pupil at a rearward portion thereof, the diameters of lens elements at a front portion thereof become larger.
Although there are restrictions as mentioned above in developing a projector apparatus, a projector apparatus employing a DMD as a light bulb is understood to be more advantageous than others not employing a DMD, in order to attain further miniaturization. Therefore currently, portable and compact apparatuses with a DMD, such as a data projector, etc., have been widely used.
In order to miniaturize the projector apparatus itself, there is obviously a high demand for the projector lens system to be miniaturized, and there is also a demand for an increase in the number of functions. Moreover, the projector's various aberrations should be corrected to satisfy the performance of a DMD. Furthermore, from the viewpoint of convenience, there are demands for zooming function with a large zoom ratio, and for having a large angle-of-view at the wide-angle extremity.